Thyroxine (T4), Free

Free Thyroxine (T4) has been the most used indice in assessing thyroxine production and thyroid function in conjunction with the thyroid stimulating hormone (TSH). Yet, maternal physiological changes in pregnancy may make this marker less accurate. In this article, we'll cover how pregnancy affects free T4 values, common causes, and the research that supports its use as a lab value in maternity care. 

Free T4 Testing

​Thyroxine (T4), free is a test that measures only the free T4 (FT4) in the blood sample.

In serum, 99.98% of T4 is bound to binding proteins, and 0.02% of T4 is free or unbound. Free T4 is the active form of the hormone.

Studies have shown that automated free T4 is often overestimated in pregnancy. They may underdiagnose those with subclinical hypothyroidism.

According to a 2002 ACOG bulletin on thyroid disease, TSH and FT4 or FTI testing should be performed in pregnant women with suspected hyperthyroidism or hypothyroidism.

Free T4 in Pregnancy

In the first trimester, there is a slight increase in FT4 levels, with a decrease after this initial phase of between 10% and 15%.

High Free T4 in Pregnancy

​Hyperthyroidism is defined as a suppressed TSH with elevated FT4. If left untreated in pregnancy, there is an increased risk of miscarriage, stillbirth, low birth weight, and preterm delivery. Maternal complications include thyroid storm and a fivefold increase in the risk of preeclampsia and congestive heart failure.

Most Common Causes of High Free T4 in Pregnancy

• Gestational Transient Thyrotoxicosis (GTT)
• Autoimmune Thyroid Disease (Grave's Disease)
• Medication Overdose
• Oxidative Stress

Low Free T4 in Pregnancy

Women with hypothyroidism have an increased risk of infertility, miscarriage, spontaneous abortion, stillbirth, placental abruption, hypertension, anemia, and postpartum hemorrhage.
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Hypothyroxinemia in the first trimester is associated with potential neurological damage in the child.

​Hypothyroxinemia is defined as FT4 <2.5th percentile of the gestational reference range with a normal TSH. 

Most Common Causes of Low Free T4 in Pregnancy

• Autoimmune Thyroid Disease (Hashimoto's)
• Iodine Deficiency
• Iron Deficiency
• Other Nutritional Deficiencies (vitamin A, vitamin D, zinc, magnesium)
• Low Progesterone
• Low Protein Diet

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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